How can I make an animation interpolate linearly between two keyframes?
The rotation will use a bezier interpolation by default (ease in and ease out). This can be viewed and changed in the Graph Editor.

Open up the graph editor, (make sure you have you're object selected), then select both keyframes for the X axis and press T while having your mouse in the graph editor window. This will bring up a menu with different keyframe interpolation modes to chose from, Select Linear and the rotation will have a constant rotation.